On the cover: Developmental mechanisms generate interspecies hybrids that cannot propagate, as alluded to by the artist's depiction of the mythical Minotaur's labyrinth on the cover. However, even socially na瘀攀 animals rarely mate with members of other species. The mechanisms that control such behavioral reproductive isolation, an important tenet of evolution, are not well understood. As depicted on the cover, male flies tap other flies with their foreleg prior to attempting to mate. Fan et al. (pp. 89–102) show that foreleg removal permits males to mate with flies of other species. Activation of foreleg neurons expressing the chemoreceptor Gr32a is necessary and sufficient to inhibit interspecies courtship by D. melanogastermales. Gr32a is required to detect aversive hydrocarbons found on the cuticle of other drosophilid species, and one such hydrocarbon, z-11-pentacosene, is visible on the female fly on the cover. Strikingly, female flies utilize a Gr32a-independent mechanism to reject males of other species, with the cover depicting the female fly rejecting the male who is tapping her. Cover art by Julie Newdoll.

Genetically hard-wired neural mechanisms must enforce behavioral reproductive isolation because interspecies courtship is rare even in sexually na瘀攀 animals of most species. We find that the chemoreceptor Gr32a inhibits male D. melanogaster from courting diverse fruit fly species. Gr32a recognizes nonvolatile aversive cues present on these reproductively dead-end targets, and activity of Gr32a neurons is necessary and sufficient to inhibit interspecies courtship. Male-specific Fruitless (FruM), a master regulator of courtship, also inhibits interspecies courtship. Gr32a and FruM are not coexpressed, but FruM neurons contact Gr32a neurons, suggesting that these genes influence a shared neural circuit that inhibits interspecies courtship. Gr32a and FruM also suppress within-species intermale courtship, but we show that distinct mechanisms preclude sexual displays toward conspecific males and other species. Although this chemosensory pathway does not inhibit interspecies mating in D. melanogaster females, similar mechanisms appear to inhibit this behavior in many other male drosophilids.